Erasure coding implementation : high level description

[Loic Dachary <loic@dachary.org>]

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Hi Sage,

Paraphrasing what you suggested today : 

The logic for writing a stripe ( i.e. all the chunks created by the erasure encoding function for a given object or part of a given object if it exceeds the maximum size of a stripe ) for a single object is going to be done in a way that is not the same as what we currently have for replicated objects. The object is consistent when all chunks ( or at least K if K+M ) are committed to disk. It may make sense to start writing all the chunks in parallel and when they are acknowledged, send a pg_log event that says : now switch to this new version of the object. To avoid ending up with chunks that are partially for one version of the object and other chunks partially for another version of the object and we can't repair any of them. 

I will try to sketch the path for implementing the erasure coding ( including the above ) by adding to https://github.com/dachary/ceph/blob/wip-4929/doc/dev/osd_internals/erasure-code.rst

Cheers

-- 
Loïc Dachary, Artisan Logiciel Libre
All that is necessary for the triumph of evil is that good people do nothing.


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